WIRELESS COMMUNICATION SYSTEM, BASE STATION, TERMINAL DEVICE, AND WIRELESS COMMUNICATION METHOD
In a communication system in which a communication is performed between a base station and one or more terminals by using one or more communication channels, a control element unit is defined by T OFDM symbols and F subcarriers. A second combination of T and F is determined for a first combination of T and F such that the number of modulation symbols (one OFDM symbol x one subcarrier) included in the control element unit defined by the second combination is closest to the number of modulation symbols included in the control element unit defined by the first combination.
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The present invention relates to a communication system in which communication is performed between a base station and one or more terminals by using one or more communication channels.
BACKGROUND ARTAt present, 3GPP (3rd Generation Partnership Project), which is a standards body, performs a standardizing task of a wireless network adopting a new wireless system under the name of LTE (Long Term Evolution). This wireless system adopts OFDMA (Orthogonal Frequency Division Multiple Access) for a downlink (direction from a base station to a terminal) and defines a plurality of communication channels and control channels on a frequency axis.
As a content of the control signal, information as to which portion of a downlink data signal area (see
Further, the CCE can be divided into a lower constituent element, that is, a control element unit (hereinafter, CEU) configured by a plurality of adjacent subcarriers. In Nonpatent Document 1, for example, the definition for a unit equivalent to the CEU (“control resource unit”) is disclosed. The definition is shown in
Nonpatent Literature 1: “Downlink L1/L2” Control Signaling: Multiplexing, Configuration and Logical Receiver Model”, 3GPP Contribution R1-070930, Chapter 2.1, p. 1-2
DISCLOSURE OF INVENTION Problem to be Solved by the InventionThe above conventional technique describes the definition of the CEU (in Nonpatent Document, it is called “control resource unit”) in which only three OFDMA symbols are used, and has no mention of other cases. The present invention is directed to perform CEU definition in the remaining cases, and an object of the invention is to harmonize among various cases of respective number of OFDM symbols to be used.
Means for Solving ProblemThe present invention is for a wireless communication system in which a base station notifies one or more terminal devices of a control signal for each subframe defined by a number of OFDM symbols that is a unit in a time axis direction and a number of subcarriers that is a unit in a frequency axis direction by using an orthogonal frequency division multiple access (OFDMA) system, and one modulation symbol is defined by one subcarrier and one OFDM symbol and a plurality of pilot signals used by the terminal device for estimating a state of a transmission channel between the terminal device and the base station are allocated in a predetermined subcarrier number cycle A for modulation symbols at identical positions in the time axis direction within a subframe. The wireless communication system includes a control-element-unit defining unit that defines, taking an integer F as an integral multiple of A, a control element unit that is a first unit of a constituent element of a control channel through which the base station transmits the control signal by T OFDM symbols and F subcarriers, and for a first combination of T and F, determines a second combination of T and F such that a number of modulation symbols included in the control element unit defined by the second combination is closest to a number of modulation symbols included in the control element unit defined by the first combination.
Effect of the InventionAccording to the present invention, a frequency width of a control element unit (CEU) in any number of OFDM symbols to be used is set to an integral multiple of a repetition cycle of a reference signal, and thus there is an effect that the demodulation performance in a terminal remains the same regardless of the number of OFDM symbols in the CEU.
100 base station
101 buffer unit
102 scheduler unit
103 data encoding/modulating unit
104 CCE deciding unit
105 control-signal generating unit
106 OFDM mapping unit
107 transmitting unit
108 receiving unit
109 received-signal determining unit
110 control-information analyzing unit
111 demodulating/decoding unit
112 ACK/NACK generating unit
200 terminal
201 receiving unit
202 CCE-number determining unit
203 control-signal decoding/detecting unit
204 demodulating/decoding unit
205 control unit
206 ACK/NACK generating unit
207 data encoding/modulating unit
208 buffer unit
209 allocation requesting unit
210 signal multiplexing/selecting unit
211 downstream-quality measuring unit
212 transmitting unit
BEST MODE(S) FOR CARRYING OUT THE INVENTION First EmbodimentA first embodiment is described with reference to FIG.
1 to
A wireless system used in the first embodiment adopts an orthogonal frequency division multiple access (hereinafter, OFDMA) for a downlink (direction from a base station to a terminal), and defines a plurality of communication channels and control channels on a frequency axis.
The reference signal, which is a prescribed signal pattern, enables estimation of a transmission channel state from a signal change upon reception, and is finally utilized for a demodulating operation of the control signal or the data signal. Numeric portions of R1 to R4 are mapped with a plurality of transmitting antenna numbers of the base station in a one-to-one correspondence, and R1 to R4 are each reference signal transmitted from the antennas 1 to 4. There is a case that the number of transmitting antennas is less than four. In this case, the part of Rn (n=2 or 3 or 4) corresponding to the missing antenna can transmit the data signal or the control signal without transmitting the reference signal.
As a content of the control signal, information as to which portion of the downlink data signal area (see
LTE defines a unit that is called a control channel element (hereinafter, “CCE”). When one of the units is used or two or more thereof are combined, one control channel is configured. In an example of
While
The number of CCEs present in one subframe is notified to the terminal. In examples of
The relation between each of the units is described orderly as follows.
- Plurality of adjacent subcarriers→control element unit (CEU)
- Predetermined number of CEUs not always adjacent→control channel element (CCE)
- At least one CCE→control channel
The number of subcarriers is set forth according to a system bandwidth, for example, ranging from 72 subcarriers in a system bandwidth of 1.25 megahertz to 1200 subcarriers in a system bandwidth of 20 megahertz. Based on the relation, it can be said that the both CEU and CCE are constituent elements of the control channel. The CEU is a first constituent element of the control channel in the present invention, and the CCE is a second constituent element in the present invention.
A characteristic of the present embodiment is described below. In this embodiment, a definition of the control element unit (CEU) when the number of OFDM symbols is other than three, particularly when it is one or two, is described.
(1) In any number of OFDM symbols to be used, a frequency width of the CEU is an integral multiple of a repetition cycle of a reference signal. That is, in a case that one OFDM symbol is used in
(2) Even when the number of OFDM symbols to be used increases, the frequency width is so adjusted that the number of configuring modulation symbols of CEU is not greatly differed. That is, in a case that one OFDM symbol is used in
When the (1) is implemented, any CEU provides the same positional relation between modulation symbol carried thereon with the control signal and the symbols of reference signals (R1 and R2 or R1 to R4). Accordingly, in view of performing a demodulation of the control signal based on a transmission channel state presumed from the reference signal, an effect that the demodulation performance remains the same even when any CEU is used is provided. This eliminates necessity of considering as to which CEU is used when the base station transmits the control signal, and thus the process can be simplified. Moreover, an error rate of control signal has a specific target value, and thus taking into account that control is performed in order that the respective CCEs (that is, CEUs) are brought close to the target value, equality in demodulation performance between the CEUs is very important in terms of process simplification.
When the (2) is implemented, it becomes possible to sufficiently increase the number of CEUs configuring the CCE even when the number of OFDM symbols to be used is changed. Thus, it is possible to bring out the frequency diversity effect described above.
In the present embodiment, the (1) and (2) are both established, and when three subcarriers (between R1 to R2 or minimum repetition unit) are applied during the use of the three OFDM symbols of
The following is a summary of CEU sizes (number of modulation symbols) and the number of effective modulation symbols of
- (a) During use of one OFDM symbol: CEU size=12, number of effective modulation symbols=8
- (b) During use of two OFDM symbols: CEU size=12, number of effective modulation symbols=8
- (c) During use of three OFDM symbols: CEU size=9, number of effective modulation symbols=7
When the six subcarriers are adopted during the use of the one OFDM symbol in (a), the CEU size is six modulation symbols, and thus the difference from the nine modulation symbols is the same as 12 modulation symbols in a case of 12 subcarriers, and the number of effective modulation symbols, however, is four. Thus, the difference from (c) during the use of the three OFDM symbols becomes large.
In
In the above explanations, the OFDM mapping unit 106 decides the configuration of the CEU as shown in
A signal received at a receiving unit 108 is distinguished into the control signal and the upstream data signal at a received-signal determining unit 109 by using previously decided use-frequency information and use-time information of each signal. A control-information analyzing unit 110 distinguishes the control signal into an upstream resource allocation request, downlink quality information, and ACK/NACK information, and notifies each of these to the scheduler unit 102. All of these pieces of information are necessary for scheduling. The upstream data signal is processed at a demodulating/decoding unit 111. Based on a process result, an ACK/NACK signal is generated at an ACK/NACK generating unit 112. The OFDM mapping unit 106 maps the ACK/NACK signal to an appropriate location.
At the terminal 200, a CCE-number determining unit 202 determines the signal received at a receiving unit 201 as an existence candidate position of the CCE (that is, CEU). Based on the information, a control-signal decoding/detecting unit 203 decodes/detects the control signal destined for itself from the control signal area.
In a case of detection, if the downlink allocation information is detected, a data decoding operation at a demodulating/decoding unit 204 is instructed, and if the uplink allocation information is detected, a control unit 205 is notified. A result processed at the demodulating/decoding unit 204 is notified to an ACK/NACK generating unit 206 at which the ACK/NACK signal is generated. At the controller, an encoding/modulating technique for an upstream data transmission is instructed to a data encoding/modulating unit 207.
As described above, each terminal is notified in advance of a plurality of control channel candidates to be received from the network side (the candidates are probably different depending on each terminal).
Further, upstream data is accumulated in a buffer unit 208. An allocation requesting unit 209 determines the transmission necessity of a resource allocation request from a data accumulating situation and a current uplink allocating situation. A signal multiplexing/selecting unit 210 selects any one of or multiplexes a plurality of the encoded data signal, the ACK/NACK signal, the resource allocation request signal, and line-quality information from a downstream-quality measuring unit 211, and transmits the same item from a transmitting unit 212.
Explanations of the first embodiment have exemplified a case of 12 subcarriers during the use of the one OFDM symbol, six subcarriers during the use of the two OFDM symbols, and three subcarriers during the use of the three OFDM symbols. However, the combination of the number of OFDM symbols to be used and the number of subcarriers is not limited thereto. For example, it is possible to adopt 24 subcarriers during the use of the one OFDM symbol, 12 subcarriers during the use of the two OFDM symbols, and six subcarriers during the use of the three OFDM symbols. As long as the conditions (1) and (2) are satisfied, other combinations can be made. This configuration also applies to other embodiments.
As a basic operation, the first embodiment has described a content that complies with a wireless communication method for which a standardizing task is currently performed under the name of LTE (Long Term Evolution) in 3GPP (3rd Generation Partnership Project), which is a standards body. However, a communication method having contents other than this can also be applied.
Moreover, while the first embodiment has described an example in which the OFDMA system is used, the present invention is not limited to the OFDMA system, and other transmission systems, which are two-dimensionally defined by a frequency axis direction and a time axis direction, can be used.
Second EmbodimentWhile the first embodiment has described a relation between the number of subcarriers of one control element unit (CEU) and the number of OFDM symbols, a second embodiment describes an example in which numbers of modulation symbols used for transmitting the control signal between differently shaped control element units are matched.
In the following explanations, a position of one modulation symbol is expressed by (X, Y), where an X direction denotes a frequency axis direction and a Y direction denotes a time axis direction in
In this case, a “distance” between the two modulation symbols is defined as a “value when the number of modulation symbols is counted in a frequency direction (subcarrier direction) and a time direction (OFDM symbol direction) from one modulation symbol to the other modulation symbol (no counting can be made in oblique directions)”. When the value is larger, the two modulation symbols are “farther”. In other words, when positions of the two modulation symbols are at (X1, Y1) and (X2, Y2), respectively, a distance L of the two modulation symbols is defined by: L=|X1−X2|+|Y1−Y2|. When L is larger, the distance is “farther”.
In
By implementing the (3), the number of CEUs configuring the CCE becomes the same all the time irrespective of the number of OFDM symbols to be used, and thus a receiving process at each terminal is facilitated. Moreover, when the (4) is implemented, an effect of prevention of deterioration of a demodulation performance is provided. Generally, the transmission channels for the other modulation symbols are estimated by using an estimate value of a transmission channel change obtained when receiving the reference signal. Thus, an estimation error about the modulation symbol far from the reference signal becomes larger, and as a result, the demodulation performance is deteriorated. Accordingly, implementing the (4) is very important in terms of not deteriorating the modulation performance.
In
The selection of the non-transmission modulation symbol in the above explanations is performed at the OFDM mapping unit 106 shown in
Further, it is necessary to feed back whether the upstream data transmission is correctly received, to the terminal from the base station. That is the ACK/NACK signal. In the present embodiment, the non-transmission modulation symbol not used for the control signal transmission can be used for the ACK/NACK signal transmission. In this case, it is not necessary to use another frequency region for the ACK/NACK signal transmission, and thus an effect of effectively utilizing a frequency band is achieved.
Third EmbodimentThe first and second embodiments have described an example in which a plurality of control element units (CEUs) configuring the control channel element (CCE) exist in the same time zone (the same OFDM symbol is occupied). The third embodiment describes an example in which the definition of the CEU is different from those in the first and second embodiments and there include some of a plurality of CEUs configuring the CCE existing in different time zones.
Specifically, in a case of the use of the three OFDM symbols in
In the second OFDM symbol in
As described above, the demodulation performance of the modulation symbol depends upon the distance from the reference signal. When only the CEUs positioned at the third OFDM symbol are collected to configure the CCE, the receiving quality greatly differs from the receiving quality provided by the CCE configured by collecting only the CEUs positioned at the first OFDM symbol. Accordingly, by using a CCE configuring method shown with reference to
The above embodiments have described a case that the number of subchannels of one frequency group is 12, the number of frequency groups is six, and a total number of subcarriers is 72. However, it is only an example, and the number of subcarriers changes depending on each system frequency range, and thus the number of subcarriers of one frequency group or the number of frequency groups can be set to any value other than that in the example.
The decision of CCEs in the above explanation is made by the OFDM mapping unit 106 shown in
Thus, in the present embodiment, the CEUs are repeatedly arranged at least in the frequency axis direction. By a plurality of CEUs at different positions on the frequency axis, the CCE as a second unit of the constituent element of the control channel is configured, and from a plurality of CCEs, the control channel corresponding to a specific terminal device is configured.
Claims
1-13. (canceled)
14. A wireless communication system in which a base station notifies one or more terminal devices of a control signal for each subframe defined by a number of orthogonal frequency division multiplexing symbols that is a unit in a time axis direction and a number of subcarriers that is a unit in a frequency axis direction by using an orthogonal frequency division multiple access system, and one modulation symbol is defined by one subcarrier and one orthogonal frequency division multiplexing symbol and a plurality of pilot signals used by the terminal device for estimating a state of a transmission channel between the terminal device and the base station are allocated in a predetermined subcarrier number cycle A for modulation symbols at identical positions in the time axis direction within a subframe, the wireless communication system comprising:
- a control-element-unit defining unit that defines, taking an integer F as an integral multiple of A, a control element unit that is a first unit of a constituent element of a control channel through which the base station transmits the control signal by T orthogonal frequency division multiplexing symbols and F subcarriers, and for a first combination of T and F, determines a second combination of T and F such that a number of modulation symbols included in the control element unit defined by the second combination is closest to a number of modulation symbols included in the control element unit defined by the first combination.
15. The wireless communication system according to claim 14, wherein
- out of the modulation symbols included in the control element unit, a modulation symbol to which no pilot signal is allocated is set to an effective modulation symbol, and
- the control-element-unit defining unit determines the second combination such that a number of effective modulation symbols included in the control element unit defined by the second combination is closest to a number of effective modulation symbols included in the control element unit defined by the first combination.
16. The wireless communication system according to claim 15, wherein when the number of effective modulation symbols in the first combination and the number of effective modulation symbols in the second combination do not match, the element defining unit sets a required number of effective modulation symbols in a combination whichever has a larger number of effective modulation symbols as a non-transmission modulation symbol that is not used for a signal transmission, thereby matching the numbers of effective modulation symbols in the first combination and the second combination.
17. The wireless communication system according to claim 16, wherein the element defining unit determines a position of the non-transmission modulation symbol as a position of a modulation symbol farthest from a pilot symbol to which a pilot signal is allocated.
18. The wireless communication system according to claim 14, wherein the element defining unit repeatedly arranges the control element units in the frequency axis direction, configures a control channel element that is a second unit of the constituent element of the control channel from a plurality of control element units, and configures a control channel corresponding to a specific terminal device from one or more of the control channel elements.
19. The wireless communication system according to claim 14, wherein
- T=3 and F=3×N in the first combination, where N is a natural number, and
- T=2 and F=6×N or T=1 and F=12×N in the second combination.
20. A wireless communication system in which a base station notifies one or more terminal devices of a control signal for each subframe defined by a number of orthogonal frequency division multiplexing symbols that is a unit in a time axis direction and a number of subcarriers that is a unit in a frequency axis direction by using an orthogonal frequency division multiple access system, and one modulation symbol is defined by one subcarrier and one orthogonal frequency division multiplexing symbol and a plurality of pilot signals used by the terminal device for estimating a state of a transmission channel between the terminal device and the base station are allocated in a predetermined subcarrier number cycle A for modulation symbols at identical positions in the time axis direction within a subframe, the wireless communication system comprising:
- a control-element-unit defining unit that defines, taking an integer F as an integral multiple of A, a control element unit that is a first unit of a constituent element of a control channel through which the base station transmits the control signal by a predetermined number of orthogonal frequency division multiplexing symbols and F subcarriers, repeatedly arranges defined control element units at least in a frequency axis direction, configures a control channel element that is a second unit of the constituent element of the control channel from a plurality of control element units at different positions on a frequency axis, and configures a control channel corresponding to a specific terminal device from the control channel elements.
21. A wireless communication method used in a wireless communication system in which a base station notifies one or more terminal devices of a control signal for each subframe defined by a number of orthogonal frequency division multiplexing symbols that is a unit in a time axis direction and a number of subcarriers that is a unit in a frequency axis direction by using an orthogonal frequency division multiple access system, and one modulation symbol is defined by one subcarrier and one orthogonal frequency division multiplexing symbol and a plurality of pilot signals used by the terminal device for estimating a state of a transmission channel between the terminal device and the base station are allocated in a predetermined subcarrier number cycle A for modulation symbols at identical positions in the time axis direction within a subframe, the wireless communication method comprising:
- control-element defining including defining, taking an integer F as an integral multiple of A, a control element unit that is a first unit of a constituent element of a control channel through which the base station transmits the control signal by a predetermined number of orthogonal frequency division multiplexing symbols and F subcarriers, arranging repeatedly defined control element units at least in a frequency axis direction, configuring a control channel element that is a second unit of the constituent element of the control channel from a plurality of control element units at different positions on a frequency axis, and configuring a control channel corresponding to a specific terminal device from the control channel elements.
Type: Application
Filed: Feb 4, 2008
Publication Date: May 13, 2010
Applicant: MITSUBISHI ELECTRIC CORPORATION (Chiyoda-ku, Tokyo)
Inventor: Noriyuki Fukui (Tokyo)
Application Number: 12/531,545
International Classification: H04L 27/28 (20060101);